The invention relates to improvements in a hydraulic brake booster for a vehicular brake system, which is actuated together with a travel spring by a brake pedal, which further is provided with a valve that controls both the interconnection of a storage source with a booster cylinder that includes a booster piston in the brake booster as well as the interconnection of the booster cylinder with a relief station. Further the brake booster includes a hydraulic dual-circuit main cylinder having a double piston for the sealing of the primary and of the secondary side and in which the booster piston is constructed as a stepped piston.
For a better understanding of this invention, reference is now made to the Leiber patent application Ser. No. 703,799, filed July 9, 1976, which is also assigned to the assignee of the present application.
On the one hand, when such brake boosters are used for wheel lock-up prevention, various problems arise. When, for example, the double pistons in the hydraulic main cylinder are used as reference pistons between the rear axle brake circuit equipped with a lock-up protection switch valve and the front axle brake circuit, then the booster piston push rod which enters the primary chamber of the main cylinder, when driven hard by large pedal forces at low reserve pressures, interferes with the free mobility of the double pistons. On the other hand, the wheel lock-up prevention function is no longer required upon the loss of the reserve pressure because the then unassisted force of the foot no longer suffices to cause the brakes to lock.